Quick Answer
A system boiler heats water and stores it in a separate hot water cylinder, while a heat only boiler, also called a regular or conventional boiler, needs both a hot water cylinder and a cold water storage tank, usually in the loft. A combi boiler heats water on demand with no cylinder or tank at all. Which one suits a property depends mainly on hot water demand and available space.
What Is a System Boiler
A system boiler has most of its major components, including the pump and expansion vessel, built into the boiler unit itself, and it heats water that is then stored in a separate hot water cylinder for use later. Unlike a heat only boiler, it does not need a separate cold water tank in the loft, since the system is sealed and pressurised. This makes it a common choice for homes with more than one bathroom, where several taps or showers might be used at once.
What Is a Heat Only (Regular) Boiler
A heat only boiler, sometimes called a regular or conventional boiler, needs both a hot water cylinder and a separate cold water storage tank, typically installed in the loft, which feeds the cylinder by gravity. This older style of system suits homes that already have this setup in place, particularly older properties, since replacing it with a different boiler type can mean removing tanks and re-routing pipework at extra cost.
What Is a Combi Boiler
A combi, or combination, boiler heats water directly from the mains on demand, with no cylinder or tank required at all. This makes it the most compact and space-efficient option, and it is the most commonly fitted type in smaller UK homes with one bathroom, though hot water flow can drop if more than one outlet is used simultaneously.
Comparison Table
| System Boiler | Heat Only Boiler | Combi Boiler | |
|---|---|---|---|
| Hot water cylinder needed | Yes | Yes | No |
| Cold water tank needed | No | Yes, usually in the loft | No |
| Space required | Moderate, cylinder only | Most, cylinder plus tank | Least, no storage needed |
| Best suited to | Multiple bathrooms, good simultaneous flow | Older properties with existing tank setup | Smaller homes, one bathroom |
| Simultaneous hot water use | Good, since water is stored | Good, since water is stored | Can drop if multiple outlets used at once |
Which Type Do You Have? How to Tell
- If you have a hot water cylinder, usually in an airing cupboard, and a tank in the loft, you have a heat only boiler
- If you have a hot water cylinder but no separate tank in the loft, you likely have a system boiler
- If you have no cylinder and no tank, and hot water appears instantly from the boiler, you have a combi boiler
- Your boiler’s model number, often printed on the front casing, can also be checked against the manufacturer’s website to confirm the exact type
Switching Between Types
Converting from a heat only or system boiler to a combi, or the reverse, is possible but adds significant cost and disruption, since it typically involves removing or adding a cylinder and tank and re-routing pipework. According to 2026 UK installation data, converting from a regular boiler to combi typically costs £800 to £1,500 in labour alone, compared with £500 to £900 for a straightforward like-for-like swap of the same boiler type. Most homeowners replacing a boiler stick with the same type already fitted, unless there is a specific reason, such as needing more simultaneous hot water capacity, to switch.
Related Reading
Getting quotes for a new installation? See our guide on boiler fitting labour-only costs.
Frequently Asked Questions
Is a system boiler better than a combi boiler?
Neither is universally better, since a system boiler suits homes needing strong simultaneous hot water flow, while a combi suits smaller homes prioritising space and simplicity.
Can I convert a heat only boiler to a combi boiler?
Yes, though it typically costs £800 to £1,500 in labour alone, since it involves removing the tank and cylinder and re-routing pipework.
Do system boilers need a loft tank?
No, a system boiler only needs a hot water cylinder, not a separate cold water tank in the loft.
Which boiler type is most common in new UK homes?
Combi boilers are the most commonly fitted type in smaller UK homes with one bathroom, due to their compact size and lack of storage requirements.
How do I know which boiler type I currently have?
Check for a hot water cylinder and a loft tank; having both means a heat only boiler, a cylinder alone means a system boiler, and neither means a combi.